Method of resin sealing permanent magnets in laminated rotor core

ABSTRACT

A method comprising: a first process of placing a laminated rotor core in a preheating device to preheat the laminated core; a second process of removing the preheated laminated core from the preheating device and disposing the laminated core between upper and lower dies of a resin sealing apparatus; a third process of pressing the laminated core by the upper and lower dies and liquefying resin material in resin reservoir pots by heating; and a fourth process of ejecting the liquefied resin material from the pots into the magnet insertion holes by plungers inserted and moving vertically in the pots and thermally curing the resin material. The method improves efficiency of resin sealing the permanent magnets in the laminated core.

CROSS REFERENCE RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.11/596,212, filed on Nov. 14, 2006, currently pending, which is a 371 ofInternational Application No. PCT/JP2006/311162 filed on May 29, 2006,which claims the benefit of priority from the prior Japanese PatentApplication No. 2006-003867, filed Jan. 11, 2006, the entire contents ofwhich are incorporated herein by references.

TECHNICAL FIELD

The present invention relates to a method of resin sealing permanentmagnets in a laminated rotor core by fixing the permanent magnets witha-resin material.

BACKGROUND ART

Conventionally, as one of methods for fixing permanent magnets to alaminated rotor core used in a motor, a method of fixing permanentmagnets with resin material has been proposed. Such a method isdisclosed, for example, in Japanese Patent Application Laid-OpenPublication No. 2002-34187, comprising the steps of: placing a laminatedrotor core in a resin sealing apparatus (also referred to as resinfilling apparatus) having upper and lower dies, the laminated rotor corebeing formed by stacking a plurality of core pieces and having aplurality of magnet insertion holes for insertion of permanent magnetsaround a central axial hole; supplying liquid resin material from resinreservoir pots formed in the upper die into the magnet insertion holesrespectively having the permanent magnets therein; and heating thelaminated rotor core to thermally cure the resin material.

The laminated rotor core is formed by a stack of core pieces blanked outfrom an electromagnetic steel sheet. Therefore, the temperature of theresin material is reduced when the resin material melted by heating isfilled into the magnet insertion holes, which disadvantageously reducesfluidity of the resin material. Accordingly, the conventional method hasdifficulty in filling the resin material into the magnet insertion holesand also in curing the resin material filled in the magnet insertionholes.

To overcome the problems, the laminated rotor core is disposed in theresin sealing apparatus having the upper and lower dies and is heated toa melting temperature of the resin material by heaters incorporated inthe upper and lower dies. Subsequently, the liquid resin material havinga favorable fluidity is supplied into the magnet insertion holes and iscured. This countermeasure allows the permanent magnets inserted in themagnet insertion holes to be retained firmly by the resin material.

However, since the melting temperature of the resin material (e.g., anepoxy resin) is as high as around 170° C., for example, it takes severaldozens of minutes for the laminated rotor core to reach the meltingtemperature of the resin material. Until the core reaches the meltingtemperature, filling of the resin material into the magnet insertionholes must be waited. Accordingly, filling operation of the resinmaterial is greatly affected by preheating operation of the core by theheaters of the upper and lower dies, which greatly reduces operatingefficiency of filling of the resin material (productivity of the resinsealing apparatus).

The present invention has been made to overcome the above disadvantagesof the prior art, and it is an object of the present invention toprovide a method of resin sealing permanent magnets in a laminated rotorcore, wherein filling operation of the resin material is immediatelyconducted without being affected by the preheating operation of thelaminated core, and by which the operating efficiency of filling of theresin material is improved compared with conventional methods.

DISCLOSURE OF INVENTION

To attain the above object, the present invention provides a method ofresin sealing permanent magnets in a laminated rotor core, comprising: afirst process of placing the laminated rotor core in a preheating deviceto preheat the laminated rotor core, the laminated rotor core beingformed by a stack of a plurality of core pieces and having a pluralityof magnet insertion holes for insertion of the permanent magnets arounda central axial hole; a second process of removing the preheatedlaminated rotor core from the preheating device and disposing thelaminated rotor core having the permanent magnets inserted thereinbetween upper and lower dies of a resin sealing apparatus; a thirdprocess of pressing the laminated rotor core by the upper and lower diesand liquefying resin material in a plurality of resin reservoir pots byheating, the resin reservoir pots being formed in either one of theupper and lower dies so as to extend to a surface thereof that contactsthe laminated rotor core; a fourth process of ejecting the liquefiedresin material from each of the resin reservoir pots by a plunger tofill corresponding one or more of the magnet insertion holes with theresin material and thermally curing (solidifying) the resin material,the plunger being inserted and moving vertically in the each of theresin reservoir pots.

In the method according to the present invention, the laminated rotorcore is placed and preheated in the preheating device before beingdisposed between the upper and lower dies of the resin sealing apparatusused for filling the resin material into the magnet insertion holes.Accordingly, filling the resin material into the magnet insertion holescan be immediately started by removing the preheated laminated rotorcore from the preheating device and disposing the core between the upperand lower dies. As a result, the filling operation of the resin materialis less affected by progress of preheating of the laminated rotor coreand efficiency of filling operation of the resin material into thelaminated rotor core is greatly improved compared with a conventionalcase where preheating of the laminated rotor core and filling of theresin material are performed in the same device.

Furthermore, since the laminated rotor core is preheated, liquefying ofthe resin material (liquefying of tablet-shaped raw material of theresin material) and filling and curing thereof are performed efficientlyafter the laminated rotor core is disposed between the upper and lowerdies, and product quality is improved compared with conventionalmethods.

In the method according to the present invention, it is preferable thatthe resin reservoir pots in either one of the upper and lower dies ofthe resin sealing apparatus are formed in a region located radiallyinward or outward with respect to the magnet insertion holes so that theresin material is filled into the magnet insertion holes from the resinreservoir pots via resin passages formed in either one of the resinsealing apparatus and the laminated rotor core. In this instance, theliquefied resin material in each of the resin reservoir pots is suppliedto corresponding one or two (three or more according to circumstances)of the magnet insertion holes via the resin passages, which allowssimplification of the apparatus. In addition, the resin reservoir potsare preferably formed penetrating either one of the upper and lowerdies.

In the method according to the present invention, it is preferable thatthe preheating of the laminated rotor core in the first process isperformed with the permanent magnets inserted in the magnet insertionholes. Consequently, preheating of the laminated rotor core and of thepermanent magnets can be carried out simultaneously. Depending on thetype of the permanent magnets, magnetism of the permanent magnets may bedecreased by heat. When such permanent magnets are used, the laminatedrotor core and the permanent magnets may be heated separately. In thiscase, the permanent magnets are heated to an appropriate temperature(e.g., in a range of 100 to 160° C.) and then are inserted in the magnetinsertion holes of the laminated rotor core heated to a highertemperature. The preheating temperature of the laminated rotor core maybe equal to a melting temperature of the resin (e.g., in a range of 150to 180° C.), or may be within a band of plus or minus 20° C. of themelting temperature. Higher preheating temperatures of the laminatedrotor core would increase fluidity of the resin material in thelaminated rotor core, thereby shortening the filling time and improvingproductivity. Lower preheating temperatures of the laminated rotor corewould reduce deformation of the laminated rotor core caused by heating,but may cause incomplete filling of the resin material depending on thestructure of the laminated rotor core. Therefore, it is preferable toset an appropriate preheating temperature according to the shape andheight of the laminated rotor core.

In the method according to the present invention, the preheating of thelaminated rotor core in the first process may be performed without thepermanent magnets inserted in the magnet insertion holes, and thepermanent magnets may be inserted in the magnet insertion holes aftercompletion of the preheating of the laminated rotor core. In thisinstance, because the permanent magnets are not directly heated by aheat source, deterioration of the permanent magnets due to excessiveheating is prevented.

In the method according to the present invention, it is preferable thatthe preheating of the laminated rotor core in the first process isperformed by any one or more of an electric heater, induction heatingand hot air.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram of a resin sealing apparatus used in amethod of resin sealing permanent magnets in a laminated rotor coreaccording to a first embodiment of the present invention.

FIG. 2 is a plan view of a laminated rotor core to which the method isapplied.

FIG. 3 is an explanatory diagram of the method.

FIG. 4 is an explanatory diagram of a preheating device used in themethod.

FIG. 5 is a plan view of a laminated rotor core to which the method ofresin sealing permanent magnets in a laminated rotor core according to asecond embodiment of the present invention is applied.

FIG. 6 is a partly sectional side view of the laminated rotor coreillustrated in FIG. 5.

FIG. 7 is a plan view of a laminated rotor core to which the method ofresin sealing permanent magnets in a laminated rotor core according to athird embodiment of the present invention is applied.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the accompanying drawings, embodiments of the presentinvention will be described for a more complete understanding of theinvention.

As illustrated in FIGS. 1 through 4, a method of resin sealing permanentmagnets in a laminated rotor core according to a first embodiment of thepresent invention includes a step of preheating a laminated rotor core(also referred to as a rotor) 13 in the vicinity of a prescribedtemperature before resin sealing permanent magnets 11 respectively in aplurality of magnet insertion holes 12 formed on a circumferential partof the laminated rotor core 13, thereby improving efficiency of asubsequent resin sealing operation. The resin sealing of the permanentmagnets 11 is effected as follows. The laminated rotor core 13 havingthe permanent magnets 11 in the respective magnet insertion holes 12 isclamped with the upper and lower dies 14 and 15 of a resin sealingapparatus 29, and resin material 17 is filled into the magnet insertionholes 12 from resin reservoir pots (hereafter also simply referred to aspots) 16 to fix the permanent magnets 11. The pots 16 are provided tothe upper die 14 so as to correspond to the respective magnet insertionholes 12. Hereafter, the method of resin sealing permanent magnets to alaminated rotor core will be fully described in detail.

First, a preparatory process is conducted for preparing the laminatedrotor cores (hereafter also simply referred to as laminated cores) 13 tobe put in preheating devices 18.

Each of the laminated cores 13 is formed by sequentially stacking aplurality of core pieces prepared by blanking an electromagnetic steelsheet into an annular shape. The electromagnetic steel sheet has athickness of, e.g., 0.2 mm or more and 0.5 mm or less. The stacked corepieces are fixed, for example, by one or both of caulking joint andwelding joint.

The laminated core 13 thus formed is placed on a carrier tray 19 removedof dust, etc. beforehand. The permanent magnets 11 having a lengthslightly shorter than a depth of the magnet insertion holes 12 arerespectively inserted in the magnet insertion holes 12 formed around anaxial hole 10 in the center of the laminated core 13. After theinsertion of the permanent magnets 11, a magnet detector (not shown)confirms whether the permanent magnets 11 are placed in the magnetinsertion holes 12 or not. The carrier tray 19 has a rectangularplate-shaped mounting portion 21 and a rod-shaped guide member 22. Alower surface 20 of the laminated core 13 contacts the mounting portion21. The guide member 22 is provided upright in a central part of themounting portion 21 and to fit in the axial hole 10 of the laminatedcore 13. A reference numeral 10 a in FIG. 2 denotes rotation stoppers(projections) for preventing rotation of the laminated core 13.

Next, the carrier trays 19 whereon the laminated cores 13 arerespectively positioned are conveyed to the preheating devices 18 by aconveyance rail 23 as shown in FIG. 3. The laminated cores 13 togetherwith the carrier trays 19 are respectively loaded into the preheatingdevices 18 and are preheated to a melting temperature of the resinmaterial 17.

(First Process)

The laminated cores 13 conveyed by the conveyance rail 23 arerespectively placed in the plurality of (in this embodiment, three)preheating devices 18 disposed laterally to the conveyance rail 23.After the laminated cores 13 are preheated to the melting temperature ofthe resin material 17 in the preheating devices 18, the preheatedlaminated cores 13 together with the carrier trays 19 are respectivelyremoved from the preheating devices 18 and are conveyed to a subsequentprocess. The resin material may be a thermosetting resin such as anepoxy resin used conventionally in semiconductor manufacturing. Themelting temperature of an epoxy resin is about 170° C. Accordingly, whenan epoxy resin is used as the resin material, the preheating temperatureof the laminated core 13 is set to around 170° C., or to a temperaturewithin a band of plus or minus 20° C. of 170° C., preferably within aband of plus or minus 10° C. of 170° C. Furthermore, the number of theheating devices 18 (precisely, the number of stages for preheating thelaminated rotor cores) represented by “n” is preferably an integer notexceeding a number obtained by a formula (T2/T1), where “T1” representsa total processing time of the resin sealing apparatus and “T2”represents a total processing time of the preheating device. Thereby,the preheating devices and the resin sealing apparatus can be operatedefficiently.

As shown in FIG. 4, each of the preheating devices 18 comprises a lowerheating unit 24 for placing the carrier tray 19 thereon and an elevatingmeans (e.g., a jack) 26 provided to a fixed mount 25 for elevating andlowering the lower heating unit 24. The preheating device 18 furthercomprises an upper heating unit 27 located above the laminated core 13elevated to an upper limit position by the elevating means 26 and alateral heating unit 28 for surrounding lateral surface of the laminatedcore 13. Divided vertically into two sections, the lateral heating unit28 is openable and closeable by moving the sections in the horizontaldirection, centering around the laminated core 13. Thereby, thelaminated core 13 is easily set in or removed from the preheating device18.

The heating units 24, 27 and 28 of the preheating device 18 arerespectively provided with electric heaters, by which the laminated core13 is heated. The laminated core 13 may also be heated by inductionheating or hot air, or by a combination of any two or more of anelectric heater, induction heating and hot air.

The preheating device in this embodiment is designed to preheat thelaminated core together with the carrier tray. However, the preheatingdevice may be configured so that only the laminated core is preheated.Although the case of employing a plurality of the preheating devices hasbeen described, only one preheating device may be used. In thisinstance, the preheating device is preferably made to have a capacity toprocess a plurality of the laminated cores. Furthermore, a tunnel-likepreheating device may be provided to the conveyance rail, whereby thelaminated cores are preheated in the preheating device while the carriertrays holding the laminated cores are conveyed by the conveyance rail.

Here, the laminated core is preheated in a state where the permanentmagnets are inserted in the respective magnet insertion holesbeforehand. Alternatively, the laminated core may be preheated withoutthe permanent magnets inserted in the magnet insertion holes, and thepermanent magnets may be inserted in the magnet insertion holes aftercompletion of the preheating of the laminated core.

Next, each of the preheated laminated cores 13 removed from thepreheating devices 18 is conveyed with the carrier tray 19 to the resinsealing apparatus 29 via the conveyance rail 23 and is disposed in theresin sealing apparatus 29. (Second Process)

As shown in FIG. 1, the resin sealing apparatus 29 comprises, forexample, the lower die 15 for holding the carrier tray 19 whereon thelaminated rotor core 13 is placed and the upper die 14 mounted over thelaminated core 13. The lower die 15 is capable of moving up and down andthe upper die 14 is elevated with the elevation of the lower die 15. Theupper die 14 has the plurality of pots 16 for raw material (alsoreferred to as tablets) of the resin material 17 to be placed therein.The upper die 14 has the pots 16 in a region located radially inwardwith respect to the magnet insertion holes 12 of the laminated core 13.The upper die 14 also has resin passages (also referred to as runners)30 at the bottom thereof for leading the melted resin material 17 inliquid form into the magnet insertion holes 12.

In FIG. 2, the pots 16 and the resin passages 30 are illustrated inimaginary line (chain double-dashed line). The pots 16 are formedpenetrating vertically the upper die 14. Namely, the pots 16 areextended to the bottom of the upper die 14.

The resin sealing apparatus 29 includes a fixed mount 31, a plurality ofplungers 32 for pressuring the resin material 17 in the pots 16 of theelevated upper die 14, and stoppers 33 for retaining the elevated upperdie 14 in an upper limit position. The plungers 32 are providedpenetrating through the fixed mount 31. The fixed mount 31 is fixedlydisposed so that a gap to be a working space for insertion of the resinmaterial 17 is formed between the upper die 14 in the lower limitposition and the fixed mount 31.

The fixed mount 31 is attached to an upper fixed plate 34. The lower die15 is placed on an elevating plate 37 that vertically moves along fourguideposts 36 connecting the upper fixed plate 34 and a lower fixedplate 35. Inside the fixed mount 31, a heating means (not shown) isprovided for preheating the plungers 32, thereby allowing easy ejectionof the resin material 17 from the pots 16 and eliminating thermalexpansion difference between the fixed mount 31 and the upper die 14 toavoid misalignment between the plungers 32 and the pots 16.

The elevating plate 37 is allowed to move up and down by a lower-dieelevating means (e.g., jack) 38 provided to the lower fixed plate 35.The plungers 32 inserted respectively in the pots 16 are elevated orlowered simultaneously in the pots 16 by a plunger driving means 39provided to the upper fixed plate 34.

The laminated rotor core 13 disposed between the upper and lower dies 14and 15 of the resin sealing apparatus 29 having such a structure ispressed with the upper and lower dies 14 and 15, and the raw material ofthe resin material 17 in the pots 16 is heated by the upper die 14 so asto be liquefied. (Third Process)

Subsequently, the liquefied resin material 17 is forced out of the pots16 into the corresponding magnet insertion holes 12 by the plungers 32via the resin passages 30, and the resin material 17 is thermally cured.(Fourth Process) Here, the resin material 17 in liquid form ejected fromthe pots 16 is filled into the magnet insertion holes 12 through theresin passages 30, i.e., through between the bottom of the upper die 14and the surface of the laminated core 13.

The temperature of the laminated core 13 at the time of filling of theresin material 17 is preferably equal to the preheating temperature ofthe laminated core 13 by the preheating device 18 or a temperaturewithin a band of plus or minus 20° C. of the preheating temperature.Consequently, for example, liquefying, filling and curing of the resinmaterial are efficiently conducted, and furthermore, deformation of thelaminated rotor core is prevented.

As described above, the laminated core 13 is preheated. Therefore, theresin material can be filled and cured in the magnet insertion holes 12in a shorter time by further heating the laminated core 13 filled withthe resin material by the heating means respectively provided in theupper and lower dies 14 and 15. The resin material may be heated usingany heating device other than the resin sealing apparatus 29.

The laminated core 13 having the magnet insertion holes 12 wherein theresin material 17 is filled and cured in this way is conveyed with thecarrier tray 19 by the conveyance rail 23 and is cooled by a coolingdevice (e.g., spot cooler) to lower the temperature of the heated core13. Subsequently, the resin material 17 overflowed from the magnetinsertion holes 12 and cured on the surface of the laminated core 13 isremoved. The side of the laminated core 13 removed of the overflowedresin material 17 is further polished when necessary, and then a totalthickness of the manufactured laminated core 13 is measured. When thethickness of the laminated core 13 meets a target thickness, rustprevention oil is sprayed on the surface of the laminated core 13 andthe laminated core 13 is shipped as a product.

As described above, since the preheating of the laminated core 13 andthe filling of the resin material 17 are performed separately (with useof separate devices), the filling operation of the resin material 17 canbe immediately conducted by the resin sealing apparatus 29 without beingaffected by preheating operation of the laminated core 13. Consequently,the working efficiency of the resin filling operation is improvedcompared with conventional methods.

Next, the method of resin sealing permanent magnets in a laminated rotorcore according to a second embodiment of the present invention will bedescried only for parts different from the method according to the firstembodiment of the present invention. A resin sealing apparatus having anupper die 40 whereon no resin passage is formed may be used as shown inFIGS. 5 and 6.

Here, resin passages (also referred to as runners) 44 for leading theresin material 17 in liquid form from resin reservoir pots 42 on theupper die 40 into magnet insertion holes 43 in a laminated rotor core 41are formed at an upper end portion of the laminated rotor core 41.

The resin passages may be formed only in a core piece on a top of thelaminated rotor core 41 or in a plurality of core pieces including thetop core piece of the laminated rotor core 41. The number of the corepieces formed with the resin passages may be decided according to athickness of the core pieces. In this embodiment, the resin passages 44are formed by two core pieces, one on the top of the laminated rotorcore 41 and the other adjoining the top core piece.

A reference numeral 45 in FIG. 5 denotes rotation stoppers (projections)for the laminated rotor core 41.

In the above embodiments, the heating means is provided to each of theupper and lower dies. Alternatively, the heating means may be providedonly to the upper die.

Furthermore, the plurality of resin reservoir pots are formed in theregion located radially inward with respect to the magnet insertionholes of the laminated core. Alternatively, the pots may be provided ina region located radially outward with respect to the magnet insertionholes. Although the case where the resin reservoir pots are provided inthe upper die of the resin sealing apparatus has been described, thepots may be provided to other parts of the resin sealing apparatus, forexample, to the lower die. When the pots are formed in the lower die,the heating means for liquefying the raw material of the resin materialis provided at least to the lower die.

Referring now to FIG. 7, the method of resin sealing permanent magnetsin a laminated rotor core according to a third embodiment of the presentinvention will be descried only for parts different from the methodaccording to the first and second embodiments of the present invention.

In this embodiment, the resin material is supplied from four resinreservoir pots 48 through resin passages 49 and 50 to eight magnetinsertion holes 47 respectively having the permanent magnets 11 therein.The pots 48 are formed in an upper die and the magnet insertion holes 47are provided around the central axial hole 10 of a laminated rotor core46. Namely, the resin material is supplied from each of the resinreservoir pots 48 to a plurality of (two or more of) the magnetinsertion holes 47. The resin passages 49 and 50 in this instance may beprovided in the upper die or on an upper surface of the laminated rotorcore 46. Decreasing the number of the resin reservoir pots in this wayallows simplification of the resin sealing apparatus.

Implementation of the present invention is not limited to the aboveembodiments, and various modifications may be made without departingfrom the scope or spirit of the present invention. Therefore, thepresent invention includes any method of resin sealing permanent magnetsin a laminated rotor core according to the present invention embodied bythe combination of a part or all of the above embodiments andmodifications.

INDUSTRIAL APPLICABILITY

In the method of resin sealing permanent magnets in a laminated rotorcore according to the present invention, the laminated rotor core ispreheated for a sufficient time before being disposed between the upperand lower dies of the resin sealing apparatus used for filling themagnet insertion holes with the resin material. Consequently, thelaminated rotor core can be processed without occurrence of thermaldeformation.

Furthermore, provision of a plurality of devices for preheating thelaminated rotor core to one resin sealing apparatus allows continuousoperation of the resin sealing apparatus, thereby improving efficiencyof the resin sealing operation of the laminated rotor core.

1. A method of resin sealing permanent magnets in a laminated rotor core, comprising: a first process of placing and preheating the laminated rotor core in a preheating device, the laminated rotor core being formed by a stack of a plurality of core pieces and having a plurality of magnet insertion holes for insertion of the permanent magnets around a central axial hole; a second process of removing the preheated laminated rotor core from the preheating device and disposing the laminated rotor core having the permanent magnets inserted therein between upper die and lower die of a resin sealing apparatus; a third process of pressing the laminated rotor core by the upper die and lower die and liquefying resin material in a plurality of resin reservoir pots by heating, the resin reservoir pots being formed in either one of the upper die and lower die so as to extend to a surface thereof that contacts the laminated rotor core; a fourth process of ejecting the liquefied resin material from each of the resin reservoir pots by a plunger to fill corresponding one or more of the magnet insertion holes with the resin material and curing the resin material, the plunger being inserted and moving vertically in the each of the resin reservoir pots; and a fifth process of cooling the laminated rotor core by a cooling device, the laminated rotor core filled with the cured resin material, wherein the resin reservoir pots penetrate the upper die or lower die in which the resin reservoir pots are formed.
 2. The method of resin sealing permanent magnets in a laminated rotor core as defined in claim 1, wherein the laminated rotor core is conveyed on a carrier tray.
 3. The method of resin sealing permanent magnets in a laminated rotor core as defined in claim 1, wherein the resin reservoir pots are formed in a region located radially inward or outward with respect to the magnet insertion holes so that the resin material is filled into the magnet insertion holes from the resin reservoir pots via resin passages formed in either one of the resin sealing apparatus and the laminated rotor core.
 4. The method of resin sealing permanent magnets in a laminated rotor core as defined in claim 2, wherein the resin reservoir pots are formed in a region located radially inward or outward with respect to the magnet insertion holes so that the resin material is filled into the magnet insertion holes from the resin reservoir pots via resin passages formed in either one of the resin sealing apparatus and the laminated rotor core.
 5. The method of resin sealing permanent magnets in a laminated rotor core as defined in claim 2, wherein the preheating of the laminated rotor core in the first process is performed with the permanent magnets inserted in the magnet insertion holes.
 6. The method of resin sealing permanent magnets in a laminated rotor core as defined in claim 2, wherein the preheating of the laminated rotor core in the first process is performed without the permanent magnets inserted in the magnet insertion holes, and the permanent magnets are inserted in the magnet insertion holes after completion of the preheating of the laminated rotor core. 